Conventionally, a cold cathode tube has been used as a light source for a backlight in a display such as an LCD. The cold cathode tube requires a high frequency and high voltage of up to about 2000 Vrms when lighting. Therefore, in a display using the cold cathode tube, safety on insulation for the generated high frequency and high voltage has to be ensured, which requires special attention to careful packaging, insulation, prevention of noise, or handling.
In order to eliminate the need of such attention, it has been considered to use, as the light source instead of the cold cathode tube, a light emitting element such as: (i) a semiconductor laser which does not require such high frequency and high voltage, or (ii) a light emitting diode (hereinafter referred as LED).
A structure of a known lighting device (semiconductor light emitting device) using such a light emitting element is illustrated in FIG. 11. In FIG. 11, a concave portion 69 is provided so that (i) light reflection can be obtained in a package body 58, (ii) one or plural light emitting element(s) 51 is (are) disposed in the concave portion 69, and (iii) a wiring pattern(s) 60 and the light emitting element(s) 51 are connected using an Au wire(s) 54, respectively.
Further, a lighting device having a structure illustrated in FIG. 12 is also known. In the lighting device shown in FIG. 12, the light emitting elements 51 are mounted in the concave portion 69, respectively. The concave portion 69 having a cup shape is provided in a resin substrate 67, in which the wiring patterns 60 are inserted by molding. Further, using the Au wires 54, the light emitting elements 51 are connected to the wiring patterns 60, respectively. Then, the concave portion 69 is sealed with an epoxy resin or the like. There has also been a lighting device having a structure in which electric wiring is wired on a surface of the resin substrate 67 without using the wiring patterns 60.
However, when a plurality of the light emitting elements 51 are disposed, it is impossible to mount all the light emitting elements 51 at the center of the concave portion 69 because a bottom surface of the concave portion 69, where the light emitting elements 51 are to be mounted, generally has a round shape. Therefore, the light emitting elements 51 are mounted on points, i.e., vertexes of a regular triangle or isosceles triangle centered on the center of the concave portion 69. Note that, it is possible to arrange one of the light emitting elements 51 to be disposed at the center point of the concave portion 69 and other light emitting elements 51 to be shifted from the center point.
In a lighting device using fluorescent material for light emission, after a blue color or ultraviolet light emitting element(s) 51 is(are) mounted inside the concave portion 69, a transparent resin is injected into and over the concave portion 69 and hardened. The transparent resin may be an epoxy, a silicone, or the like, in which one or plural fluorescent material(s) required for light emission with an intended color(s) is(are) mixed.
In a backlight unit (backlight device) employing such a lighting device, as illustrated in FIG. 13, a plurality of lighting devices 76, formed with the foregoing method, are disposed so that light from the lighting devices 76 is incident from the side surface to the inside of an optical waveguide 77. The optical waveguide 77 is used for converting the light from dot emission to surface emission. Further, as to the backlight unit, it has been widely known that brightness is enhanced and unevenness of the light is reduced by (i) installing reflecting sheets 78 to side- and bottom-surfaces of the optical waveguide 77 where no lighting device 76 is disposed, and (ii) stacking a diffusion sheet 80, a luminance improving film 79, or the like over a light emitting surface of the optical waveguide 77 as illustrated in FIG. 14.
However, according to the conventional structure, as illustrated in FIGS. 15(a) through 15(c), when a plurality of the light emitting elements 51 are mounted in the concave portion 69, it is impossible to dispose all of the light emitting elements 51 at the center of the concave portion 69. This causes variations in distances between the light emitting elements 51 and a sidewall of the concave portion 69 having light reflecting efficiency or light shielding efficiency. Accordingly, as illustrated in FIG. 16, luminance distributions of light beams emitted from the light emitting elements 51, i.e., directional characteristics 51a and 51b, have various gradients with respect to a center axis 69a of the concave portion 69 (see Japanese Unexamined Patent Publication No. 294838/2000 (Tokukai 2000-294838, publication date: Oct. 20, 2000) and Japanese Unexamined Patent Publication No. 303936/2003 (Tokukai 2003-303936, publication date: Oct. 24, 2003).
Further, in such a structure that the light emitting elements 51 are positioned on vertexes of a triangle, but not on a linear line (disclosed in Japanese Unexamined Patent Publication No. 307818/1999 (Tokukai 1999-307818, publication date: Jan. 5, 1999), the luminance distributions have gradients in y-direction as well as x-direction. Accordingly, as illustrated in FIG. 17, directional characteristics 51a through 51c are formed in various directions, i.e., in the x-direction and the y-direction with respect to the center axis 69a, due to the positional differences of the light emitting elements 51. When the light emitting elements 51 have three emission colors of RGB, mixture of the colors will be perceived as different colors from different angles.
When the conventional lighting device (semiconductor light emitting device) is employed as a light source in a backlight device for an LCD, as illustrated in FIG. 18, the light emitting elements 51 exhibit various directional characteristics to the optical waveguide 77, which converts light into surface emission. This is caused by the positional differences of the light emitting elements 51 as described above. Further, incident light beams 75, which are incident from the light emitting elements 51 on the side surface of the optical waveguide 77, travel in different directions at different angels from a normal to the side surface. Accordingly, luminance efficiency for the light incident on the optical waveguide 77 is reduced.
When light beams of RGB three colors are emitted, the light beams tend to have unevenness in colors due to mixture of the colors. Further, when two or more fluorescent materials are used, a total amount of the fluorescent materials is increased and their transmittance is deteriorated, so that the light brightness is reduced. When a fluorescent material having a long excitation wavelength is used, the fluorescent material absorbs light, which is emitted from another fluorescent material, for excitation. Thus, luminance efficiency is reduced.